A pacemaker-mediated tachycardia (PMT), also referred to as endless loop tachycardia, occurs when a pacemaker paces the ventricles at inappropriately fast rates for sustained periods of time. PMT occurs when a ventricular event occurs at a time during which the connective tissue between the atrium and ventricle can transmit retrograde electrical signals from the ventricle to the atrium. The conduction of the ventricular signal to the atrium provides a spurious electrical signal in the atrium that is considered to be a natural atrial event by the pacemaker. The pacemaker senses the spurious retrograde atrial signal and then paces the ventricle at a programmed AV time period following the signal. The paced ventricular signal is conducted to the atrium and is again erroneously detected by the pacemaker as a natural atrial event. The pacemaker therefore continues to pace the ventricle at a relatively high rate defined by the sum of the programmed AV interval and the retrograde conduction time between the ventricle and atrium. The high rate is sustained indefinitely by the pacemaker, because retrograde conduction ensures that the pacemaker detects what appear to be high rate atrial events and tracks these spurious atrial events by generating corresponding high rate ventricular paces.
PMT can be caused by retrograde conduction following a premature ventricular contractions (PVC), a right ventricular (RV) pace can produce retrograde conduction back up to the atrium. In response to the retrograde conduction, an atrial sensed event to be detected by the implanted device which in turn causes an RV pace to be deliver at the programmed sensed atrioventricular (SAV) interval. This produces a cycle of inappropriately fast pacing by the device called PMT, as described in U.S. Pat. No. 4,554,920 to Baker et. al.
Existing algorithms for detection of PMT may be able to achieve improved PMT detection, especially for patients experiencing high intrinsic rates. High intrinsic rates can lead to inappropriate PMT intervention (e.g. extending post-ventricular atrial refractory period (PVARP) for one cycle) which can lead to dropped beats or the interruption of CRT pacing. It is therefore desirable to develop devices and methods that are configured to avoid, eliminate, terminate or reduce PMTs.